In French Pat. Nos. 1 459 336 and 94472/1 459 336 there is described an installation to measure the distance limiting visibility in a fog, comprising a series of lights disposed for example along a landing strip of an airfield or another measuring area, an optical observation device projecting the image of one or a plurality of lights of the series onto an observation screen, for example the target of a television imaging tube or one of a plurality of photoelectric cells and a motorized control device to set the variable focal lens of the optical device on a selected light of said series and to simultaneously actuate the lights viewed on the optical device. The lights may be illuminated continuously or in a blinking manner.